Compositions based on random propylene copolymers, process for their manufacture, and heat-sealable multilayer sheets comprising them

ABSTRACT

Compositions based on propylene polymers (compositions (C)) comprising: a) from 61 to 74% by weight of a random propylene copolymer (copolymer (A)) comprising from 8 to 16% by weight of monomer units derived from 1-butene and less than 0.5% by weight of monomer units derived from ethylene, and b) from 39 to 26% by weight of a random propylene copolymer (copolymer (B)) comprising from 35 to 50% by weight of monomer units derived from 1-butene and from 0 to 1% by weight of monomer units derived from ethylene. Process for the manufacture of these compositions.

[0001] Compositions based on random propylene copolymers, process fortheir manufacture, and heat-sealable multilayer sheets comprising themThe present invention relates to compositions based on random propylenecopolymers. It relates more particularly to compositions exhibiting botha relatively high melting temperature and a very low sealing temperaturecomprising two random propylene copolymers with different compositions.The present invention also relates to a process for the manufacture ofthese compositions. Finally, it relates to heat-sealable multilayersheets, at least one heat-sealable layer of which comprises the saidcompositions.

[0002] It is known to use random propylene co- or terpolymers comprisinga few % by weight of ethylene and/or of 1-butene for the manufacture ofthe heat-sealable layers (known as “sealing layers”) of multilayersheets for packaging, the central layer of which is based on acrystalline propylene polymer, which are generally biaxially oriented(known as “BOPP films”).

[0003] One disadvantage of these random co- and terpolymers is the factthat, at a given sealing temperature (hereinafter known more conciselyas ST), the melting temperature (hereinafter known more concisely as MT)is too low for use on certain machines for the manufacture of sheets,thus resulting in problems of adhesion to the rollers of these machines.

[0004] Attempts have been made to overcome these advantages by usingcompositions comprising two propylene copolymers comprising variableamounts of monomer units derived from ethylene and/or 1-butene.

[0005] Mention may be made, as examples of documents disclosing suchcompositions, of:

[0006] the document EP-A-0 560 326, which discloses semi-crystallinecompositions based on propylene copolymers comprising:

[0007] from 20 to 60% by weight of a random copolymer (A) of propyleneand of a C₄₋₁₀ α-olefin comprising from 1 to 10% by weight of C₄₋₁₀α-olefin;

[0008] from 40 to 80% by weight of a random copolymer (B) of propyleneand of a C₄₋₁₀ α-olefin comprising from 15 to 40% by weight of C₄₋₁₀α-olefin;

[0009] the document EP-A-0 263 718, which discloses weakly crystallinecompositions based on propylene copolymers comprising:

[0010] from 60 to 95% by weight of a random copolymer composed of 97 to86 mol % of propylene, of 0.5 to 6 mol % of ethylene and of 2 to 13 mol% of a C₄₋₂₀ α-olefin, the molar ratio of the α-olefin to the sum of theethylene and α-olefin which are incorporated being from 0.3 to 0.9;

[0011] from 40 to 5% by weight of a random copolymer composed of 10 to90 mol % of propylene and of 90 to 10 mol % of a C₄₋₂₀ α-olefin;

[0012] the document GB 2 116 989, which discloses compositions based onpropylene copolymers comprising:

[0013] from 60 to 95 parts by weight of a propylene co- or terpolymercomprising from 75 to 99% by weight of propylene, from 0 to 7% by weightof ethylene and from 0 to 25% by weight of 1-butene;

[0014] from 40 to 5 parts by weight of a propylene co- or terpolymercomprising from 25 to 95% by weight of 1-butene, from 0 to 10% by weightof ethylene and from 5 to 75% by weight of propylene;

[0015] the document EP-A-0 679 686, which discloses compositions formedfrom two random propylene copolymers comprising:

[0016] from 75 to 95% by weight of a copolymer of propylene and of anα-olefin comprising from 3 to 27.% by weight of α-olefin;

[0017] from 5 to 25% of a copolymer of propylene and of an α-olefincomprising from 35 to 65% by weight of α-olefin.

[0018] However, the contents of C₄₋₁₀ α-olefin of the typicalcompositions described in the document EP-A-0 560 326 are too low andconsequently do not allow very low ST values to be achieved. The typicalcompositions described in the document EP-A-0 263 718 have a predominantblock necessarily comprising ethylene and consequently do not exhibit anoptimum compromise between the MT and the ST. The typical compositionsdescribed in the document GB 2 116 989 comprise an excessive amount ofcomonomers in the predominant block and consequently have an excessivelylow MT, which can present problems when they are employed for theshaping of sheets, such as problems of adhesion to the rollers of themachines used for this operation. The content of copolymer very rich inα-olefin in the compositions described in the document EP-A-0 679 686 istoo low to sufficiently lower the ST.

[0019] Specific compositions based on random propylene copolymerscomprising monomer units derived from ethylene and 1-butene which makeit possible to satisfactorily solve all the problems listed above havenot been found.

[0020] To this end, the present invention mainly relates to compositionsbased on propylene polymers (composition (C)) comprising:

[0021] from 61 to 74% by weight of a random propylene copolymer(copolymer (A)) comprising from 8 to 16% by weight of monomer unitsderived from 1-butene and less than 0.5% by weight of monomer unitsderived from ethylene, and

[0022] from 39 to 26% by weight of a random propylene copolymer(copolymer (B)) comprising from 35 to 50% by weight of monomer unitsderived from 1-butene and from 0 to 1% by weight of monomer unitsderived from ethylene.

[0023] The terms “monomer units derived from propylene”, “monomer unitsderived from 1-butene” and “monomer units derived from ethylene” will bereplaced hereinbelow by the abbreviations “C₃”, “C₄” and “C₂”respectively.

[0024] Compositions comprising higher amounts of copolymer (A) have anexcessively high ST, while lower amounts of copolymer (A) result incompositions exhibiting problems of adhesion to the rollers used duringthe manufacture of the sheets. Compositions comprising higher contentsof C₄ in the copolymer (A) have excessively low MT values. Compositionscomprising higher contents of C₄ in the copolymer (B) can give problemsof morphology or alternatively of maintenance of the reactor in the gasphase. Low contents of C₄ in the copolymers (A) and (B) result in anexcessively high ST. Higher contents of C₂ in the copolymers (A) and (B)result in an excessively low MT and a content of fractions soluble inorganic solvents which is too high for application in sheets intendedfor food packaging.

[0025] The copolymers (A) which can be used in the compositionsaccording to the invention result in a satisfactory ST and asufficiently high MT. In addition, they can be obtained undertechnically and economically acceptable polymerization conditions.

[0026] The preferred compositions according to the invention comprise atleast 62% by weight and at most 72% by weight of copolymer (A).

[0027] The copolymer (A) is generally such that the amount of C₄ is, atleast 10% by weight with respect to the said copolymer (A). Amounts ofat most 14% by weight make it possible to obtain compositions (C) havinga good compromise between the MT and ST.

[0028] The copolymers (A) used according to the invention can compriseless than 0.5% by weight of C₂. This low content of C₂ improves thesuitability for printing of the sheets manufactured from thecompositions (C). Preferably, the amount of C₂ present in the copolymers(A) is at most 0.3% by weight. The copolymers (A) not comprising C₂result in an optimum MT of the composition.

[0029] The compositions (C) according to the invention also comprisefrom 39 to 26% by weight of copolymer (B).

[0030] The amount of copolymer (B) is preferably at least 28% and atmost 38% by weight.

[0031] This copolymer (B) preferably comprises at least 38% by weight ofC₄. Good results are obtained when the amount of C₄ in the copolymer (B)is at most 45% by weight. The copolymer (B) can also comprise up to 1%by weight of C₂. The presence of C₂ makes possible the manufacture ofthe copolymers (B) with a higher yield. The copolymer (B) advantageouslycomprises less than 0.5% by weight of C₂. Copolymers (B) not comprisingC₂ are particularly well suited and result in compositions (C) having anoptimum compromise between the MT and the ST.

[0032] The compositions (C) according to the invention which giveparticularly good results comprise:

[0033] from 62 to 72% by weight of a propylene/1-butene copolymer (A)essentially comprising from 86 to 90% by weight of monomer units derivedfrom propylene and from 10 to 14% by weight of monomer units derivedfrom 1-butene, and

[0034] from 38 to 28% by weight of a random propylene copolymer (B)comprising from 38 to 45% by weight of monomer units derived from1-butene and from 0 to 0.5% by weight of monomer units derived fromethylene.

[0035] The compositions according to the invention are preferablycomposed essentially of the copolymers (A) and (B). Preferably again,the copolymers (A) and (B) comprise only monomer units derived frompropylene, 1-butene and optionally ethylene.

[0036] The compositions (C) according to the invention generally exhibita melt flow index (MFI), measured according to Standard ASTM D 1238(230° C.); load: 2.16 kg), of 0.5 to 20 g/10 min, preferably of 1 to 15g/10 min.

[0037] The MT of the compositions (C) according to the invention,measured by differential scanning calorimetry (DSC) according toStandard ASTM D 3418, is generally at least 125° C. and preferably atleast 128° C. The compositions having an MT of less than 125° C. giveproblems of adhesion to the drawing rollers in the lengthwise directionduring the manufacture of BOPP films. The MT of these compositions isgenerally at most 146° C. and more particularly at most 144° C. Thecompositions having an MT of greater than 146° C. do not allowlow-temperature sealing. Compositions (C) having an MT of 128 to 138° C.are very particularly preferred because they allow easy operation onmachines for the manufacture of BOPP films, with absence of adhesion tothe drawing rollers in the lengthwise direction, and use on high-speedpackaging machines.

[0038] The ST of the compositions (C) according to the invention isgenerally at most 100° C. and advantageously at most 98° C. Compositions(C) for which the ST is at most 96° C. are very particularly preferred.Generally, the ST of the compositions according to the invention is atleast 70° C. and more particularly at least 80° C. The ST is determinedas follows: two coextruded and biaxially oriented films with a thicknessof 25 μm are placed between the jaws of an Otto Brugger HSG/C sealingdevice with two heated jaws. A pressure of 3 bar is applied for onesecond. After cooling, a tensile test at a rate of 100 mm/min is carriedout. The ST of the temperature, expressed in ° C., at which the weldexhibits a strength of 100 g/cm. The coextruded biaxially. orientedfilms are composed of a central layer of propylene homopolymer (MFI: 2.8g/10 min) with a thickness of 23 μm and of two outer layers of thecomposition (C) according to the invention with a thickness of 1 μm. Tomanufacture these sheets, a draw ratio in lengthwise direction of 4.5 isapplied and a draw ratio in the transverse direction of 8 is applied.

[0039] The compositions (C) according to the invention exhibit both ahigh MT and a low ST. The compositions (C) according to the inventionare generally characterized in that the MT is generally at least 26° C.greater than the ST. They consequently exhibit an optimum compromisebetween sealability and processability on the lengthwise drawing rollersof machines for the manufacture of BOPP films. The MT is advantageouslyat least 30° C. greater than the ST.

[0040] Compositions (C) exhibiting an MT at least 35° C. greater thanthe ST are particularly preferred.

[0041] The DSC thermogram of the compositions (C) according to theinvention which is obtained as described hereinafter generally exhibitsthree melting peaks: a first melting peak (known hereinafter as MT1)between 115 and 145° C., a second melting peak (known hereinafter asMT2) between 70 and 130° C. and a third melting peak (known hereinafteras MT3) between 50 and 90° C. The DSC thermogram is obtained by usingthe following method: after an isotherm for 5 minutes at −10° C, thenheating from −10° C. to 220° C. at 10° C./min, followed by maintainingat 220° C. for 10 minutes, a sample of the composition (C) (in the formof a pressed film having a thickness of approximately 50 to 100 μm) iscooled to 125° C. at 20° C./minute, then to 40° C. at 1° C./minute andsubsequently to −10° C. at 20° C./minute, and is maintained at −10° C.for 5 minutes; the sample is finally reheated to 180° C. at 10°C./minute. To the best of our knowledge, the melting peak MT3 wouldoriginate from the melting of at least one fraction of the copolymer(B).

[0042] The compositions (C) according to the invention generally exhibita hot tack range from 90 to 150° C. and more particularly from 100 to145° C. The hot tack range is determined using the device represented inFIG. 1, which comprises a device (1) which makes it possible to attach,in the lengthwise direction, a strip of film (2) above an Otto BruggerHSG/C sealing device (3) with two jaws with a width of 1 cm (3A and 3B),a shaft (4) which makes possible the introduction of the strip betweenthe two jaws, and a weight (5) of 63.3 g intended to be attached to theother end of the strip. The distance between the weight and the lowerjaw is 8 cm, the lateral displacement distance of the film during theintroduction between the jaws of the sealing device is 3 cm, and thedistance between the point of attachment of the film and the upper jawis 11 cm. The hot tack range is measured as follows: a strip with awidth of 4 cm and a length of 29.5 cm of a coextruded and biaxiallyoriented film with a thickness of 25 μm obtained according to the methoddescribed hereinabove in connection with the measurement of the ST,attached by one end to the device (1) and by the other to the weight(5), is introduced, using the shaft (4), between the two jaws of thesealing device (3), which subsequently close for 0.5 s with a pressureof 5 bar. On opening the jaws, the percentage of weld which has remainedintact is measured. This measurement is made at various temperaturesbetween 70 and 160° C. with a frequency of 5° C. and the hot tack rangeis defined as being the range of temperatures within which at least 80%of the weld is observed to have remained intact after opening the jaws.

[0043] The compositions (C) according to the invention generally exhibitoptical properties, such as gloss, transparency and haze which areparticularly favourable. These properties are measured on the coextrudedand biaxially oriented films obtained as described hereinabove. Thehaze, measured according to Standard ASTM D-1003, is generally at most1.7% and at least 0.6%. The gloss, measured according to Standard ASTMD-2457 at 45°, is generally at least 78% and at most 100%. Thetransparency, determined according to Standard ASTM D-1746, isadvantageously at least 95% and at most 100%.

[0044] The compositions (C) according to the present invention generallyexhibit a hexane-soluble fraction (known hereinafter more simply asHSF), measured as indicated in “Food Drug Cosmetic Law Reports” of 1Apr. 2000; Volume 21CFR; paragraph 177.1520; method (d) (3) (ii) andexpressed as percentage by weight of fractions soluble in n-hexane at50° C. with respect to the whole of the composition, of at most 5.5% andmore particularly of at most 4.5%. They are consequently generally wellsuited to the manufacture of sealing layers for multilayer sheetsintended for the packaging of foodstuffs.

[0045] The invention also relates to a process for the manufacture ofthe compositions (C). According to a first alternative form, thecompositions according to the invention can be obtained by preparing thecopolymer (A) and the copolymer (B) separately and by mixing thesecopolymers according to any technique known for this purpose.

[0046] The copolymers (A) and (B) can be prepared in a known way bycopolymerization of propylene with 1-butene and, if appropriate, withethylene. The principle of these copolymerizations is known to a personskilled in the art.

[0047] The copolymers (A) and (B) are preferably obtained by gas-phasepolymerization. Such a process is preferred because it makes it possibleto easily incorporate large amounts of comonomer(s). Thesecopolymerizations can be carried out continuously or batchwise or by acombination of the two methods. The copolymers (A) and (B) can beblended, for example, by melt blending in an internal or external mixer.This blending is preferably carried out in a mixer of the extruder type.

[0048] According to a particularly advantageous alternative form, thecompositions according to the present invention are prepared bysuccessive copolymerizations, one of the copolymers being preparedduring a first stage and the other copolymer being prepared, in thepresence of the first, during a subsequent polymerization stage. Each ofthese stages can be carried out in the same polymerization mixture or indifferent polymerization mixtures. Preferably, the copolymer (A) isfirst prepared and subsequently the copolymer (B) is prepared in thepresence of the copolymer (A) resulting from the first stage. Aparticularly preferred method for the preparation of the composition (C)consists in successively synthesizing the copolymer (A) and then thecopolymer (B) in the presence of the copolymer (A) by gas-phasepolymerization in successive interconnected reactors operating asstirred beds or preferably as fluidized beds. The latter alternativeform gives no problem of agglomeration and results in an excellentparticle size of the compositions (C) according to the invention.

[0049] The copolymers (A) and (B) can be obtained by means of any knownsufficiently active and productive Ziegler-Natta catalytic system whichmakes it possible to polymerize propylene in a sufficientlystereospecific form and which can incorporate, in the polymer, therequired amounts of 1-butene and optionally of ethylene. The preferredcatalytic systems for preparing the compositions according to theinvention comprise:

[0050] a catalytic solid comprising, as essential components, magnesium,titanium and chlorine,

[0051] an organoaluminium compound, preferably a trialkylaluminium, veryparticularly triethylaluminium,

[0052] an electron-donating compound (external electron donor) generallychosen from alkoxysilanes of formula R¹ _(n)Si(OR²)_(4-n) in which Rrepresents a hydrocarbonaceous group comprising from 1 to 12 carbonatoms, R² represents a hydrocarbonaceous group comprising from 1 to 8carbon atoms and n is 1, 2 or 3.

[0053] The alkoxysilanes which are preferred as external electron donorare alkylalkoxy- and cycloalkylalkoxysilanes, and, among the latter, di-and trimethoxysilanes are very particularly preferred.n-Propyltrimethoxysilane, dicyclopentyldimethoxysilane andcyclohexylmethyldimethoxysilane give particularly good results.

[0054] The catalytic solids which can be used according to the inventionare well known to a person skilled in the art. They generally comprisean electron-donating compound (internal electron donor) chosen frommono- and diesters of aromatic carboxylic acids, preferably from dialkylphthalates, very particularly diisobutyl phthalate. The catalytic solidswhich can be used according to the present invention can also comprisegreater or lesser amounts of preformed polymers originating from apre-polymerization stage carried out during the synthesis of the saidsolid or from a polymerization stage directly preceding the manufactureof the compositions (C) according to the present invention.

[0055] The various constituents of the catalytic system are generallymade use of so that the atomic ratio of the aluminium of theorganoaluminium compound to the titanium of the catalytic solid is from3 to 300, preferably from 10 to 250 and very particularly from 15 to100. In addition, the molar ratio of the aluminium of theorganoaluminium compound to the electron-donating compound is generallyfrom 0.5 to 60, preferably from 1 to 50, very particularly from 2 to 30.

[0056] The other general polymerization conditions are well known to aperson skilled in the art. The temperature is generally from 20 to 150°C., preferably from 40 to 95° C., very particularly from 50 to 75° C.The polymerization is generally carried out at a pressure greater thanatmospheric pressure, preferably from 10 to 50×10⁵ Pa.

[0057] The average molecular mass of the copolymers formed can beadjusted by the addition of one or more known agents for adjusting themolecular mass, such as hydrogen, diethylzinc, alcohols, ethers andalkyl halides. Hydrogen is the most commonly used.

[0058] The required amounts of monomers and of agent for adjusting themolecular mass can be introduced continuously or batchwise into thepolymerization mixture.

[0059] The compositions (C) can optionally be subjected to one or moreknown treatments with water, an alcohol and/or a hydrocarbonaceousdiluent in order to remove the catalytic residues and/or the fractionsof low molecular weight.

[0060] The compositions (C) according to the present invention cancomprise additives known for improving the properties thereof. Theseadditives are, for example, stabilizers, neutralizing agents,lubricants, slip-promoting agents and antistatic agents. They aregenerally added by melt blending, for example during the manufacture ofgranules from the compositions (C) according to the invention. The MFIof the compositions (C) can also, if necessary, be adjusted by blending,preferably melt blending, with, for example, a peroxide.

[0061] As mentioned above, the compositions (C) according to theinvention can be used for the manufacture of heat-sealable sheets andmore particularly for the manufacture of multilayer sheets for whichthey constitute the sealing layer or layers.

[0062] It is obvious that several compositions (C) can be used in thesealing layer or layers.

[0063] The multilayer sheets generally comprise a base layer which canbe composed of any polymeric material, generally of a crystallineα-olefin polymer.

[0064] Preferably, the base sheet is composed substantially of acrystalline propylene polymer. This crystalline propylene polymer canalso be a copolymer of propylene, ethylene and/or 1-butene generallycomprising less than 8% by weight of these comonomers, preferably lessthan 5% by weight. It is obvious that the multilayer sheets cancomprise, in addition to the base sheet and the sealing layer or layerscomprising the compositions (C), other intermediate layers between thebase sheet and the sealing layer or layers.

[0065] The multilayer sheets according to the invention can be obtained:

[0066] by causing a preshaped base sheet and one or more sheetspreshaped from the compositions (C) to adhere to one another by pressingbetween heated rollers;

[0067] by coating a base sheet by means of a solution or dispersioncomprising the compositions (C) in an organic solvent, in order tolaminate the compositions (C) on the base sheet;

[0068] by extruding a layer comprising the molten compositions (C), inorder to laminate it on the base sheet;

[0069] by separately extruding the layer or layers comprising thecompositions (C) and the base sheet and by joining the molten extrudatesvia a common die;

[0070] by coextruding the layer or layers comprising the compositions(C) and the base sheet via a multichannel die.

[0071] The thickness of the multilayer sheets according to the inventionis generally from 5 to 300 μm, preferably from 10 to 170 μm. Thethickness of the sealing layer(s) based on the composition (C) isgenerally from 0.1 to 50 μm, preferably from 0.5 to 30 μm. The thicknessof the base sheet is generally from 5 to 200 μm, preferably from 10 to70 μm.

[0072] The multilayer sheets according to the invention can be usedwithout being drawn. Preferably, they are drawn monoaxially and moreparticularly biaxially, for the purpose of their orientation, aftercoating the base sheet with the layer or layers comprising thecompositions (C). This drawing can be carried out according to one ofthe known methods below:

[0073] a layer comprising the compositions (C) is extruded over apreformed base sheet and the multilayer sheet thus obtained is drawnbiaxially;

[0074] a base sheet is drawn beforehand monoaxially, while hot, in thelengthwise direction (machine direction (MD)) by means of a series ofrollers, including a metal roller, a layer of the composition (C) isextruded over this sheet, which has thus been monoaxially drawn, and themultilayer sheet thus obtained is drawn in the transverse direction(TD);

[0075] the multilayer sheet obtained by extrusion, through a common die,of the layer or layers comprising the compositions (C) and of the basesheet is drawn biaxially (in the lengthwise direction (LD) and in thetransverse direction (TD)) in two successive stages or simultaneously.

[0076] The final method is preferred.

[0077] The multilayer sheets thus obtained are appropriate for anypackaging application. They are advantageously used on high-speedpackaging machines. They are particularly well suited to the packagingof food.

[0078] The following examples serve to illustrate the invention.

[0079] In these examples, the MFI, HSF, MT, MT1, MT2, MT3 and ST values,the hot tack range and the optical properties are measured as describedabove. The C₄ content of the copolymer (A) is determined by Fouriertransform IR spectrometry, by using the absorption band at 767 cm⁻¹,with respect to a pressed film of 200 μm and is expressed as % byweight. The C₄ content of the copolymer (B) is determined from the C₄content of the composition (C), measured by nuclear magnetic resonance(NMR), the C₄ content of the copolymer (A) and the content of copolymer(B). The content of copolymer (B) is determined from the catalyticresidues of the composition (C) with respect to the copolymer (A). TheC₂ content is determined by Fourier transform IR spectrometry from theabsorption bands at 733 cm⁻¹ and 720 cm⁻¹ and is expressed as % byweight. The C₃ content is determined by the mass balance and is alsoexpressed as % by weight.

EXAMPLE 1

[0080] The preparation was carried out as follows of a composition (C)comprising (percentages by weight): 70% of a copolymer (A) and 30% of acopolymer (B). The copolymer (A) comprised 87.8% of C₃ and 12.2% of C₄and the copolymer (B) comprised 60.5% of C₃ and 39.5% of C₄. The totalcontent of C₄ in the composition (C) was 20.4%.

[0081] Each of the copolymers (A) and (B) was successively prepared intwo fluidized-bed reactors, each equipped with a gas distribution grid,connected in series and operating continuously. A gas flow comprisingnitrogen, propylene, 1-butene and hydrogen was passed through thesereactors, via a compressor, at a stable stationary concentration. Asample of the copolymer (A) was withdrawn for analysis.

[0082] The polymerization conditions are found in Table 1 below.

[0083] The catalytic system introduced into the first reactor contained:

[0084] a catalytic solid comprising 2.6% by weight of Ti and 11% byweight of diisobutyl phthalate supported on MgCl₂,

[0085] triethylaluminium (TEAL), and

[0086] n-propyltrimethoxysilane.

[0087] The characteristics of the composition (C) thus obtained arefound in Table 1 below.

[0088] A composition composed of:

[0089] 100 parts by weight of the composition (C) described above;

[0090] 0.05 part by weight of calcium stearate;

[0091] 0.05 part by weight of dihydrotalcite;

[0092] 0.201 part by weight of stabilizer composed of one third byweight of pentaerythrityltetrakis(3,5-di(tert-butyl)-4-hydroxyphenylpropionate) and of two thirdsby weight of tris(2,4-di(tert-butyl)phenyl) phosphite;

[0093] 0.101 part by weight of Crodamide® ER erucamide from Croda;

[0094] 0.126 part by weight of Sylobloc® 45 silica from Grace Davison,

[0095] was granulated in a Clextral BC45 twin-screw extruder (die at 90°C., under nitrogen).

[0096] The necessary amount of2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DBPH) was added to themixture in order to obtain granules for which the MFI was 5 g/10 min.These granules were coextruded on either side over a random propylenecopolymer comprising approximately 0.45% by weight of ethylene andcharacterized by an MFI of 2.8 g/10 min, so as to obtain a film forwhich the total thickness after biaxial orientation (4.5 times in themachine direction and 8 times in the transverse direction) was 25 μm,with 1 μm of sealing layer comprising the composition (C). Thecharacteristics of the sealing layer of the BOPP film thus obtained arefound in Table 2 below.

EXAMPLE 2

[0097] The preparation was carried out as follows of a composition (C)comprising (percentages by weight):

[0098] 63% of a copolymer (A) and 37% of a copolymer (B). The copolymer(A) comprised 88% of C₃ and 12% of C₄ and the copolymer (B) comprised59.9% of C₃ and 40.1% of C₄. The total content of C₄ in the composition(C) was 22.4%.

[0099] Each of the copolymers (A) and (B) was successively preparedaccording to the general conditions mentioned in Example 1. Thepolymerization conditions and the characteristics of the composition (C)obtained are found in Table l.

[0100] This composition (C) was granulated under the conditionsdescribed in Example 1. The granules thus obtained, the MFI of which was5 g/10 min, were used for the manufacture of a BOPP film as described inExample 1, the sealing layer of which exhibited the characteristicsmentioned in Table 2.

EXAMPLE 3R

[0101] This example is given by way of comparison.

[0102] A composition comprising only a single copolymer, containing (%by weight) 81.9% of C₃, 17.4% of C₄ and 0.7% of C₂, is prepared in thefirst of the reactors mentioned in Example 1 and according to thegeneral conditions mentioned in this example.

[0103] The polymerization conditions and the characteristics of thecomposition obtained are found in Table 1.

[0104] The composition obtained was granulated under the conditionsdescribed in Example 1, except that the amount of DBPH was adjusted inorder to obtain granules for which the MFI was 7 g/10 min, the contentof Sylobloc® 45 was 0.1 part by weight, the content of dihydrotalcitewas 0.1 part by weight, per 100 parts by weight of the composition (C),and the additions of erucamide and calcium stearate were omitted. Thegranules thus obtained were used for the manufacture of a BOPP film asdescribed in Example 1, the sealing layer of which exhibited thecharacteristics mentioned in Table 2.

[0105] The relatively low MT of this composition results in problems ofadhesion during its use as sealing layer for multilayer sheets.

EXAMPLE 4R

[0106] This example is given by way of comparison.

[0107] The preparation was carried out as follows of a compositioncomprising (percentages by weight):

[0108] 75% of a copolymer (A) and 25% of a copolymer (B). The copolymer(A) comprised 86.5% of C₃ and 13.5% of C₄ and the copolymer (B)comprised 84.3% of C₃, 4.7% of C₂ and 11% of C₄. The total content of C₄in the composition was 13% and the total content of C₂ was 1.3%.

[0109] Each of the copolymers (A) and (B) were successively preparedaccording to the general conditions mentioned in Example 1.

[0110] The polymerization conditions and the characteristics of thecomposition obtained are found in Table 1.

[0111] This composition was granulated under the conditions described inExample 1, except that the addition of the DBPH was omitted. Thegranules thus obtained, the MFI of which was 5 g/10 min, were used forthe manufacture of a BOPP film as described in Example 1, the sealinglayer of which exhibited the characteristics mentioned in Table 2.

[0112] The relatively high ST of this composition is a handicap withregard to its use as heat-sealable film on high-speed packagingmachines. TABLE 1 Unit Ex. 1 Ex. 2 Ex. 3R Ex. 4R Polymerizationconditions for (A) (A) weight % 70 63 100 75 H₂/C₃ mol/mol % 0.22 0.461.1 1.1 C₄/C₃ mol/mol % 17.33 16.52 30.2 19 C₂/C₃ mol/mol % — — 1.7 —Pressure 10⁵ Pa 31.9 32.05 36 36 Temperature ° C. 60 60 60 65 Al/Timol/mol 80 80 50 70 Al/Si mol/mol 5 5 3.5 5 Residence time hour 2.08 1.52 1.8 Characteristics of the copolymer (A) C₂ weight % 0 0 0.7 0 C₄weight % 12.2 12 17.4 13.5 (B) weight % 30 37 0 25 Polymerizationconditions for (B) H₂/C₃ mol/mol % 2.57 3.03 — 2.8 C₄/C₃ mol/mol % 72.74110 — 17 C₂/C₃ mol/mol % — — — 2.9 Pressure 10⁵ Pa 20.1 21.9 — 19Temperature ° C. 60 67 — 65 Residence time hour 2.27 2.42 — 1.8Characteristics of the copolymer (B) Polymerization conditions for (B)C₂ weight % 0 0 — 4.7 C₄ weight % 39.5 40.1 — 11 Characteristics of thecomposition (C) MFI g/10 min 2.3 3.6 3.5 4.9 C₂ weight % 0 0 0.7 1.3 C₄weight % 20.4 22.4 17.4 13 HSF weight % 3.1 3.9 46 4.5 MT ° C. 131.9133.2 117 130 MT1 ° C. 137 139 120.4 136.2 MT2 ° C. 114 114 101 113.6MT3 ° C. 78 70 — —

[0113] TABLE 2 Sealing layer comprising (C) Unit Ex. 1 Ex. 2 Ex. 3R Ex.4R MFI g/10 min 5 5 7 5 ST ° C. 95 86 92 108 Hot tack range ° C.-° C.105-142 105-135 95-135 Haze % 1.5 1.3 1.8 Gloss % 79 80 77 Clarity % 9697 95

1. Compositions based on propylene polymers (compositions (C))comprising: from 61 to 74% by weight of a random propylene copolymer(copolymer (A)) comprising from 8 to 16% by weight of monomer unitsderived from 1-butene and less than 0.5% by weight of monomer unitsderived from ethylene, and from 39 to 26% by weight of a randompropylene copolymer (copolymer (B)) comprising from 35 to 50% by weightof monomer units derived from 1-butene and from 0 to 1% by weight ofmonomer units derived from ethylene.
 2. Compositions according to claim1, comprising: from 62 to 72% by weight of a propylene/1-butenecopolymer (A) essentially comprising from 86 to 90% by weight of monomerunits derived from propylene and from 10 to 14% by weight of monomerunits derived from 1-butene, and from 38 to 28% by weight of a randompropylene copolymer (B) comprising from 38 to 45% by weight of monomerunits derived from 1-butene and from 0 to 0.5% by weight of monomerunits derived from ethylene.
 3. Compositions according to either one ofclaims 1 and 2, exhibiting a melt flow index (MFI) measured according toStandard ASTM D 1238 (230° C.; load: 2.16 kg), of 1 to 15 g/10 min. 4.Compositions according to any one of claims 1 to 3, exhibiting a sealingtemperature of at most 100° C.
 5. Compositions according to any one ofclaims 1 to 4, characterized in that the melting temperature is at least26° C. greater than the sealing temperature.
 6. Compositions accordingto any one of claims 1 to 5, exhibiting a melting peak MT3 of 50 to 90°C.
 7. Process for the manufacture of the compositions according to anyone of claims 1 to 6, by mechanical melt blending of the copolymers (A)and (B) in an internal or external mixer.
 8. Process for the manufactureof the compositions according to any one of claims 1 to 6, by successivecopolymerizations, the copolymer (A) being synthesized during the firststage and the copolymer (B) being synthesized during the second stage inthe presence of the copolymer (A).
 9. Heat-sealable multilayer sheets,characterized in that they comprise at least one sealing layer composedsubstantially of a composition (C) according to any one of claims 1 to6.
 10. Packaging material based on sheets according to claim 9.